The Zirkel UL 20 Degree bag is insulated with 850 fill DownTek insulation. Down insulation has long been renowned for its compressible properties and efficiency in delivering an excellent warmth-to-weight ratio. The shortcoming of down has been its susceptibility to the effects of moisture, losing much of its performance qualities when wet or damp. DownTek insulation utilizes nano technology and a proprietary application process to provide water-repellency and both anti-microbial and anti-bacterial protection. A nylon microfiber rip-stop shell also features a water repellent treatment and provides lightweight durability.

Insotect Flow technology is used in the Zirkel UL 20 Degree bag to help eliminate cold spots, regulate the movement of down inside the baffles and offer better body heat distribution. Strategically placed Flow Gate construction eliminates lateral and vertical down shifting by using vertical chambers with "gates" to control the position and density of the down insulation.

The Zirkel UL 20 Degree features the Big Agnes System, which integrates the pad sleeve in the design of the bag. In a traditional sleeping system, the bag lies on top of a sleeping pad. In this configuration, many people find themselves rolling off the pad at night. Plus, the insulation on the bottom of the bag is compressed by the sleeper's weight. Once compressed, the fill loses most of its insulating properties. The Big Agnes System is different, joining the bag and pad together. The pad slides into a sleeve on the bottom side of the bag, supplying insulation to the otherwise uninsulated bottom of the bag while the top and sides of the bag are insulated like a traditional sleeping bag. This design provides a secure foundation so you can roll, twist and turn freely during the night without ever rolling off your pad. It also prevents the bag from twisting and engulfing you in an uncomfortable mess. Big Agnes uses this system throughout their entire line of sleeping bags.

A number of other design features bolster the warmth and comfort of the mummy-shaped Zirkel UL 20 Degree bag. An insulated ergonomic hood, a draft tube located along the length of the zipper, and an insulated neck yoke lock in heat and keep out cold air. Integrated connection loops enable you to use the bag with a liner (sold separately). A built-in pillow pocket holds a Big Agnes Air Core Pillow (also sold separately), clothes or other pillows, providing a comfortable resting place for your head and neck.

Available in Regular (fits to 5’ 10") and Long (6’ 6") lengths, each Zirkel UL 20 Degree sleeping bag includes a nylon stuff sack and a mesh storage sack.